CFD and Thermo Mechanical Analysis on Effect of Curved vs Step Surface in IC Engine Cylinder Head

Current research in IC engines mainly focus on various methods to achieve higher efficiency and high specific power. As a single design parameter, combustion chamber peak spring pressure has increased more than before. Apart from the structural aspects of withstanding these loads, designer faces challenges of resolving thermal aspects of cylinder head. Methods to enhance the heat transfer without compromising load withstanding capability are being constantly explored. Conventional cylinder heads have got sat inner surface. In this paper we have suggested a modification in inner surface to enhance the heat transfer capability. To increase the heat transfer rate, inner same deck surface is configured as a curved and stepped surface instead of sat. We have reported the effectiveness of extend of curvature in the inner same deck surface in a different technical paper. Here, we are making a direct comparison between stepped and curved surface only. From this analysis it has been observed that curved surface reduces the ame deck temperature considerably without compromising the structural strength factors compared to step and sat surface.

N. GANESH S. BALAJI CFD AND THERMO MECHANICAL ANALYSIS ON EFFECT OF CURVED Vs STEP SURFACE IN IC ENGINE CYLINDER HEAD Current research in IC engines mainly focus on various methods to achieve higher ef ciency and high speci c power. As a single design parameter, combustion chamber peak ring pressure has increased more than before. Apart from the structural aspects of withstanding these loads, designer faces challenges of resolving thermal aspects of cylinder head. Methods to enhance the heat transfer without compromising load withstanding capability are being constantly explored. Conventional cylinder heads have got at inner surface. In this paper we have suggested a modi cation in inner surface to enhance the heat transfer capability. To increase the heat transfer rate, inner ame deck surface is con gured as a curved and stepped surface instead of at. We have reported the effectiveness of extend of curvature in the inner ame deck surface in a different technical paper. Here, we are making a direct comparison between stepped and curved surface only. From this analysis it has been observed that curved surface reduces the ame deck temperature considerably without compromising the structural strength factors compared to step and at surface.    1. Tetrahedral elements were used to mesh the model and it took 1950663 nodes and 10154632 elements to arrive at a considerable mesh quality. The material properties considered for analysis is given in table 1. CFD analysis is carried out as per the standard analysis method using ANSYS FLUENT. Coolant flow in the coolant passage was assumed to be 3D flow, steady state, incompressible turbulent flow, and the viscosity in the near wall region was also taken into account. CFD analysis was carried out using coolant at a constant inlet temperature of 120 • C. Body Temperature is obtained as output from CFD. For ThermoMechanical analysis, load due to the combustion gas peak pressure is applied on the flame deck surface considering the body temperature into account.

II. BOUNDARY CONDITIONS
Energy and turbulence model was considered for CFD analysis. Cast Iron is considered as Solid body and Water as the coolant domain. 7 m/s water inlet velocity and 393 • K is taken as input parameter for water. At outlet 1bar abs. pressure is taken as boundary condition. Simplex algorithm was used as solution method and first order up wind scheme was used. Boundary surfaces are shown in fig 2.  Assumptions: 1. On all surfaces of the cylinder heat transfer coefficient and velocity of charge is uniform. 2. This model accounts the increase in gas velocity in the cylinder during combustion. 3. Woschni's proposal that the average gas velocity should be proportional to the mean piston speed. 4. Air is the working medium. 5. Heat transfer by conduction through the walls is one-dimensional Thermo dynamic simulation model is also considered for the HTC calculation and the values obtained are given in Table  2.
These heat transfer coefficient are validated using mathematical calculation based upon Woschni equation [2].The equation is given as below: where, The heat transfer coefficient on the gas side, h Vs θ for the engine can be represented by the Fig 3. Average of the heat transfer coefficient calculated is found matching with values obtained through thermodynamics simulation. Average heat transfer coefficient considered on the gas side is 800.87 W/m 2 K.

IV. Steady State Thermal Analysis
In IC engines heat from the combustion fluctuates periodically.This high rpm makes the temperature fluctuations to penetrate about a millimeter. Depth of heat penetration is governed by the Thermal diffusivity α. Considering rated speed engine as 2600 rpm calculation for depth of heat penetration is shown in fig 4. Engine rated speed = 2600 rpm corresponding time scale = 0.023 sec Thermal di f f usivity o f cast iron α = 1e −5 m 2 /sec Penetration Depth X = √ α * t (6) Since the depth of heat penetration is small it is assumed that the temperature profile does not change with reference to time. Hence steady state thermal analysis is carried out. The heat distribution is shown

III. Grid Independent Test
One of the essential aspects of CFD simulation study is to check the grid sensitivity. The effect of grid size on the results has been studied for case-2 to prove the grid independent results. Number of grid has been reduced from 2751925 to 305030 as shown in Fig 5. Velocity and Temperature were compared for both fine and coarse mesh. It can be inferred from the figures 6 & 7 that there is no significant change in the values of velocity, temperature, which shows that the results are not sensitive to the grid.

IV. Results and Observation
CFD and thermo mechanical analysis was carried out for three different configurations. Coolant inlet velocity is taken as 7m/sec. Boundary conditions are maintained same for all the three cases. From the structural analysis it is found to withstand the peak firing pressure. Summary of CFD results for each case; velocity profile and temperature an stress distribution is given in Table 3. Configuration 3 performs better compared to other two cases in terms of low body temperature and stress distribution.