Numerical Simulation Of Flow And Heat Transfer In Cooling Channels Of Active Cooled Scramjet Engines

Table look-up methods, which provided efficient thermodynamic property reconstruction capability, were developed and incorporated into commercial CFD package FLUENT. This numerical tool could simulate flow and heat transfer of supercritical fluids efficiently. Numerical investigation of flow and heat transfer on supercritical-pressure,mildly-cracked hydrocarbon fuel in both tube and rectangular cooling channel of scramjet engine were conducted.Three-dimensional look-up table was developed to improve the efficiency of thermodynamic property evaluation in global reaction. This method is validated and is two orders of magnitude faster than SUPERTRAPP database. Heat transfer of n-decane with endothermic reaction in a tube were simulated. The effect of pressure change in the tube on thermodynamic property can be neglected.In order to improve the storage efficiency of the table look-up method significantly,block-structured adaptive mesh and adaptive triangular mesh were used to construct lookup table, respectively. Overlapping uniform index domain was developed to provide fast search capability. Constructing look-up table with both block-structured adaptive mesh and adaptive triangular mesh are more simple and efficient than with adaptive Cartesian mesh and triangulation-pattern-recognizing. When the look-up table is constructed with block-structured adaptive mesh, the storage requirement decreases by 20%-80% compared with the table constructed with uniform mesh. The storage saving increases to68%-97% when the look-up table is constructed with adaptive triangular mesh. The overlapping uniform index domain increases the searching speed by a factor of 2 and the memory by less than 37% compared with the tree data structure. The table look-up method is as efficient as ideal gas equation of state and two orders of magnitude faster than the complex thermodynamic database.Heat transfer characteristics of supercritical pressure fluid in tubes were studied numerically. Large thermodynamic property gradient appears in cross-wise direction before the bulk fluid temperature is larger than its critical point. The heat transfer characteristics of different fluids is qualitatively similar. However, fluid-to-fluid modeling in quantitative manner is still difficult and further investigation is required for a specific fluid. Two types of HTD were observed. The type I HTD, which appeared at the initial heated section, was due to the formation of the thermal boundary layer. The type II HTD, which could be identified with M-shaped velocity profile, appeared because of the reduction of turbulent diffusion. The HTD onset parameter, which was defined as the ratio of heat flux to mass flux, was obtained at different accelerations. Both types of HTD onset parameter increased almost linearly as the positive acceleration increased. The type II HTD onset parameter could not be identified when the negative acceleration was less than certain value. Correlations for the HTD onset parameter as a function of acceleration were fitted and could be adopted in the design of the active cooling system of scramjet engines.Coupled heat transfer of supercritical-pressure n-decane in asymmetrically heated rectangular cooling channels were simulated. Detail flow field under deteriorated heat transfer, effect of secondary motions, wall thermal conductivity, channel aspect ratio,and endothermic reaction on heat transfer was studied. The distorted M-shaped velocity profile can be used to identify heat transfer deterioration in asymmetrically heated rectangular channels. The secondary flow of Prandtl’s first kind, which is due to cross-wise pressure gradient in curved section, was at the order of 1.0% of the maximum axial velocity. The secondary motion of Prandtl’s second kind, which is due to Reynolds stress,was one magnitude less than that of Prandtl’s first kind. Higher wall thermal conductivity produces more uniformly distributed heat flux at both cross-wise and stream-wise direction. High fluid temperature region appeared and developed near the vertical midplane at both concave and convex side. High aspect ratio cooling channel produces lower wall temperature when the hydraulic diameters, inlet Reynolds numbers, and heat fluxes are equal, respectively. Endothermic reaction reduces the wall temperature. However,low-conversion chemical reaction almost has no influence in the flow field because the reaction appears only close to the wall.