Exhaust port challenge

Exhaust Port Challenge

In this challenge, Heat transfer through the walls of exhaust port is simulated using different mesh operations. Significance of the finer mesh is explained.

1.Conjugate heat transfer (CHT) analysis can accurately predict heat transfer by simultaneous solving all the relevant solid and flow field heat transfer processes, for example: conduction through solids, free and forced convection in the gases/fluids and thermal radiation.

The CHT approach has an advantage over other thermal analysis in that wall heat transfer coefficients and their local variations on surfaces are directly calculated within the model rather than based on simplified empirical calculations. The CHT approach therefore has benefits for those applications where heat transfer is either non-uniform or difficult to calculate empirically. The analysis type CHT allows the simulation of the heat transfer between Solid and Fluid domains by exchanging thermal energy at the interfaces between them.

Meshing

case-1: for this case, we have opted for the default mesh by Ansys. Base mesh size 0.15m. Mesh is corse and there no inflation layers involved in the mesh.

Using the default mesh size 1.3 lakh cells are formed, since the cell count is less computational time will be less as well.

Case-2: Since we have used a corser mesh in the first case we will be using a finer mesh in this case. Base mesh size is 0.15m. Mesh in the solid domain is made even more finer by introducting body sizing mesh of size 0.01m

Inorder to get presice heat transfer and flow parameters we have introduced inflation layers at the walls of the geometry. Element size is 0.005m. To get a finer and even mesh capture curvature option is turned on.

3.7 lakhs cells are formed by the mesh options we have checked.

Solution

case-1: we have choosen a steady state, pressure based solver, inflow velocity = 5m/s, inflow temperature =700k and K-ephsilon turbulence for the solution. All the parameters reached convergence at 100th iteration.

Once the solution converged this is the temperature profile of the domain. Outlet port of the exhaust port is at higher temperature compared to inlet ports this is due to the high mass flow rate.

Case-2: As we have high mesh size in this case, computational time was high as well. only 150 iterations were used for this process. Increase in mesh size changed the convergence of the solution. In case1 solution became stable by 100th iteration whereas in this case solution has not reached steady level. Few more iterations coud have yeilded stability.

Once the solution converged this is the temperature profile of the domain. Outlet port of the exhaust port is at higher temperature compared to inlet ports this is due to the high mass flow rate.

Results

case-1:

Below picture represents the temperature with the help of streamlines. High convection is recorded at the outlet section of the domain, as mass flow rate from the 4 inlet ports combined to create a very high mass flow rate region, which made presence of high mass in the outlet region.

this is a midcut plane of the outlet pipe. Wall of the solid domain is becoming hotter as the exhaust is moving towards the outlet.

Velocity contour and temperature wall convection contour tells us that the velocity and convection are high at same place.

Velocity cutplane section of the outlet pipe.

Below picture explains clearly that wall heat transfer coefficient depends on the velocity in the domain as it is high in same place.

Case-2

Below contour represents the temperature wall convection. in the label section, it is observed that the lowest temperature is high compared to the case1.

velocity of the exhuast is more by 2meters when compared to the case1.

expansion due to the bent made the velocity increase.

wall heat transfer coefficient is twice as much as case1. mesh size has greater effect on the values of heat transfer.

3.Verifiation of the HTC predictions from the simulations:

i. using the experimental data we can compare the results from the simulation. This is a classic way of verifing the results.

ii. Using the correlations methods proved to be very effecctive and are accurate as well.

iii. it can be estimated by dividing the thermal conductivety of the fluid by a length scale. The heat transfer coefficient is often calculated from the Nusselt number.

Factors on which the accuracy of the prediction depend on:

i. Physiochemical properties of fluids ( both cold and hot ) such a viscosity , density, specific heat, thermal conductivity.

ii. Geometry optimization.

iii. Mesh size (A finer mesh size produces better results)

iv. Choosing of correct models for the solution can help get better results.